University of Tennessee, Knoxville

Trace: Tennessee Research and Creative Exchange Masters Theses

Graduate School

5-2014

The Crystal Growth of Cesium Cerium Chloride Scintillator for X-Ray and Gamma-Ray Spectroscopy Applications Adam Coleman Lindsey University of Tennessee - Knoxville, [email protected]

Recommended Citation Lindsey, Adam Coleman, "The Crystal Growth of Cesium Cerium Chloride Scintillator for X-Ray and Gamma-Ray Spectroscopy Applications. " Master's Thesis, University of Tennessee, 2014. http://trace.tennessee.edu/utk_gradthes/2732

This Thesis is brought to you for free and open access by the Graduate School at Trace: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Masters Theses by an authorized administrator of Trace: Tennessee Research and Creative Exchange. For more information, please contact [email protected].

To the Graduate Council: I am submitting herewith a thesis written by Adam Coleman Lindsey entitled "The Crystal Growth of Cesium Cerium Chloride Scintillator for X-Ray and Gamma-Ray Spectroscopy Applications." I have examined the final electronic copy of this thesis for form and content and recommend that it be accepted in partial fulfillment of the requirements for the degree of Master of Science, with a major in Materials Science and Engineering. Mariya Zhuravleva, Major Professor We have read this thesis and recommend its acceptance: Charles L. Melcher, Claudia J. Rawn Accepted for the Council: Dixie L. Thompson Vice Provost and Dean of the Graduate School (Original signatures are on file with official student records.)

The Crystal Growth of Cesium Cerium Chloride Scintillator for X-Ray and Gamma-Ray Spectroscopy Applications

A Thesis Presented for the Master of Science Degree The University of Tennessee, Knoxville

Adam Coleman Lindsey May 2014

Dedication For my family and friends who instilled within me a self-respect that pushed me to achieve more than I could have alone. My older brother and two sisters have always been excellent examples to follow in life and they serve as the pillars supporting my confidence that I can make a difference in the world as they have before me. Of course none of us would be here without our parents and I must acknowledge my Mother’s love and unwavering support that have helped shape me into the man I am today. My god parents who worked for decades looking after my family during their life, and also in death, your gifts have been seeds that have grown exponentially, and I will always remember to pay this forward. I wish my late Father could be here to see my progress in the years since his passing and guide me further. He sacrificed much of his life to support my mother and siblings and I know he would be proud. My wife Lisa, who has been with me for over 9 years has been a source of inspiration, encouragement, happiness, hopefulness, and love that have carried me through times of stagnation, apprehension, sadness, anxiety, and fear. I am a greater man because of her.

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Acknowledgements I would like to acknowledge the fine advice of Dr. George Pharr, Dr. Claudia Rawn, and Dr. Veerle Keppens for providing my first consultations prior to entering my studies here at the University of Tennessee and encouraging me to pursue an advanced degree. Additional thanks go to Dr. Rawn for encouraging me to engage fellow students as a mentor, leader, and representative. I would like to acknowledge the support provided by the US Department of Homeland Security, Domestic Nuclear Detection Office under grant # 2012-DN-077-ARI067-02 for funding the project constituting the majority of my research. I would also like to acknowledge the financial support of Siemens Medical Solutions for their contributions to the Scintillation Materials Research Center as well as the Center for Materials Processing which has provided financial support to me and fellow students. Thank you to Merry Koschan who has always been a source of great advice, critique and inspiration in my research. I wish to also thank the many people at the SMRC who welcomed me to their lab and helped make it my place of work and research. I would like to extend a special thanks to my fellow students that have worked to make this lab what it is today and reminded me that I am never alone in my studies. They are Dr. Harold Rothfuss, Dr. Kan Yang, Hua Wei, Fang Meng, Sam Donnald, Luis Stand, William McAlexander, and my lab sister, Bonnie Blalock. I have shared kind and sobering words with each of you and I deeply appreciate the human element you impart to my experiences here. I wish to acknowledge the staff at UT that make my research possible. They are the glass blowers Arthur Pratt and Bo Bishop as well as the machinists Larry Smith, Danny Hackworth and Doug Fielden. Their workmanship is an integral part of solving the many engineering problems I have encountered. I wish to thank Frank Holiway for being the “man who gets things” and Randy Stooksbury for being the “man who lets us have the things the department already has”. Without these two, progress would truly be stifled. A special thank you to Dr. Charles Melcher and Dr. Mariya Zhuravleva for seeing potential in me and supporting me through what I consider to be the most challenging and rewarding few years of my life.

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Abstract The detection and identification of sources of nuclear radiation is an integral tool in defending our nation from threats of nuclear terrorism as well as enforcement of nuclear non-proliferation agreements around the globe. To improve the capabilities in this application, new detection materials surpassing the performance of existing technology utilizing sodium iodide [NaI:Tl] scintillator crystals must be developed and their production cost lowered to meet the demand for the large volumes required. A recently discovered intrinsic scintillation material in the form of crystalline cesium cerium chloride (CsCe2Cl7) has demonstrated promising performance in the detection of X-ray and gamma ray radiation. In order to assess the potential of this material to be developed into larger scale growth of crystals greater than one cubic inch in volume, research into optimizing the growth processes at smaller volumes is necessary. Single crystalline boules of CsCe2Cl7 were grown from the melt in sealed fused silica ampoules using the Bridgman method of crystal growth. A transparent growth furnace along with continuous observation apparatus were developed to aid in the investigation of the growth processes. A comparison of growth and cracking behavior under varied conditions was produced and growth protocols identified which improve crystal boule quality. Crystal quality benefits from controlling the self-seeding process through manipulation and control of critical freezing point isotherms during growth. Cracking appears to originate from aggressive detachment of the crystal from the fused silica ampoule wall while inclusions formed during growth by constitutional supercooling of the melt introduce additional crack nucleation sites through action as stress intensifiers within the bulk matrix. Reducing ampoule volume has a minor effect on cracking severity while additions of excess cesium chloride to the initial mixture produce a greater reduction in cracking. The anisotropic coefficients of thermal expansion as well as the refined crystal structure of cesium cerium chloride have been determined through single crystal Laue and temperature dependent powder X-ray diffraction pattern analyses respectively.

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Table of Contents Objective ....................................................................................................................................................... 1 Chapter 1: Introduction to Radiation Detection ............................................................................................ 2 1.1 Application.......................................................................................................................................... 2 1.2 Scintillator Based Detectors ................................................................................................................ 2 1.3 Single Crystalline Metal Halide Scintillators...................................................................................... 3 Chapter 2: Crystal Growth ............................................................................................................................ 6 2.1 Growth Mechanics and Methods ........................................................................................................ 6 2.1.1 Thermodynamics governing phase transformations .................................................................... 6 2.1.2 Analysis of the CsCl-CeCl3 Phase Diagram ................................................................................ 6 2.1.3 Methods of Bulk Crystal Growth for Metal Halides.................................................................... 8 2.2 Factors Affecting Crystal Quality ....................................................................................................... 9 2.2.1 Supercooling ................................................................................................................................ 9 2.2.2 Constitutional Supercooling....................................................................................................... 11 2.2.3 Defect Formations...................................................................................................................... 15 Chapter 3: Motivation, Method, and Equipment ........................................................................................ 17 3.1 Motivation ......................................................................................................................................... 17 3.1.1 Prior Growth Efforts .................................................................................................................. 17 3.1.2 Inconsistencies in Reported Structure of CsCe2Cl7 ................................................................... 18 3.2 Method Description .......................................................................................................................... 19 3.2.1 Single Crystal Growth of CsCe2Cl7 Using the Bridgman Method............................................. 19 3.2.2 Structure Refinement of CsCe2Cl7 ............................................................................................. 20 3.2.3 Scintillation Performance Characterization ............................................................................... 21 3.2.4 Thermal Profiling and Numerical Modeling of Growth Stations .............................................. 24 3.3 Equipment Development................................................................................................................... 25 3.3.1 Transparent Furnace Growth Station ......................................................................................... 25 3.3.2 Continuous Observation Equipment .......................................................................................... 25 3.3.3 Ampoule Design ........................................................................................................................ 28 3.3.4 Data Logging ............................................................................................................................. 29 Chapter 4: Results and Discussion.............................................................................................................. 30 4.1 Structural, Thermodynamic, and Thermo-physical Analysis of CsCe2Cl7 ....................................... 30 4.1.1 Synthesis and Phase Identification through Powder X-ray Diffraction ..................................... 30 4.1.2 Structure Refinement from Laue Diffraction Data .................................................................... 31 4.1.3 Differential Scanning Calorimetry............................................................................................. 34 4.1.4 Hygroscopicity ........................................................................................................................... 34 4.1.5 Coefficients of Thermal Expansion through Temperature Dependent PXRD........................... 35 4.2 Thermal Profiling and Numerical Modelling of the Transparent Furnace........................................ 38 4.3 Crystal Growth of CsCe2Cl7 and Observations ................................................................................. 39 4.3.1 First Attempt at Growth ............................................................................................................. 39 v

4.3.2 Growth within the Transparent Furnace .................................................................................... 44 4.3.3 Thermal Gradient and Annealing as Process Variables ............................................................. 46 4.3.4 Potential Volatilization Concerns .............................................................................................. 50 4.3.5 The Effect of Non-stoichiometric Starting Composition on Crystal Quality ............................ 53 4.4 Scintillation Performance Characterization ...................................................................................... 58 Chapter 5. Conclusion................................................................................................................................. 64 Future Outlook ........................................................................................................................................ 64 List of References ....................................................................................................................................... 66 Appendix..................................................................................................................................................... 71 Vita ............................................................................................................................................................. 73

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List of Tables Table 1. Comparison of Properties of Metal Halide Scintillators in Advanced Development ..............................4 Table 2. Comparison of Physical and Scintillation Properties of CsCe2Cl7, NaI:Tl, and Lu2SiO5:Ce ............... 17 Table 3. CsCe2Cl7 Crystal Growth Samples ............................................................................................................ 20 Table 4. Refined Structure of CsCe2Cl7 ................................................................................................................... 31 Table 5. Anisotropic Coeffiecients of Thermal Expansion of CsCe2Cl7 and other Metal Halide Scintillators .. 37 Table 7. Table of Structural, Thermophysical, and Scintillation Properties of CsCe2Cl7. .................................. 72

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List of Figures Figure 1. The Binary CsCl-CeCl3 phase diagram. ....................................................................................................7 Figure 2. Schematic of a Bridgman-Stockbarger Crystal Growth Station ........................................................... 10 Figure 3. Differential Scanning Calorimetry measurement................................................................................... 11 Figure 4. Melt inclusions caused by constitutional supercooling seen under magnification in LaBr3. .............. 12 Figure 5. Constitutional diagram. ............................................................................................................................ 13 Figure 6. The equilibrium liquidus temperature. ................................................................................................... 14 Figure 7. Protuberances along the growth interface. ............................................................................................. 14 Figure 8. Laue X-ray diffraction apparatus with 2-D detector used in this study. .............................................. 22 Figure 9. Temperature Dependent PXRD. .............................................................................................................. 23 Figure 10. Transparent furnace development. ........................................................................................................ 26 Figure 11. Continuous Observation Equipment ..................................................................................................... 27 Figure 12. The necked capillary design used for isolating a grain during self-seeding ....................................... 28 Figure 13. Comparison of measured PXRD data from CsCe2Cl7 with the available pattern from CsPr2Cl7. ... 30 Figure 14. Atomic model of the refined CsCe2Cl7 structure as viewed along the crystal axes. ........................... 32 Figure 15. The calculated powder pattern for CsCe2Cl7 with labeled hkls for angle dispersive X-rays. ........... 32 Figure 16. Measured PXRD data from CsCe2Cl7 compared with the calculated pattern. .................................. 33 Figure 17. The differential scanning calorimetry measurement of CsCe2Cl7....................................................... 34 Figure 18. The hygroscopicity curves for CsCe2Cl7 in comparison with SrI2, LaBr3, and NaI. ......................... 35 Figure 19. Temperature dependent diffraction of CsCe2Cl7. ................................................................................. 36 Figure 20. Unique angle temperature dependence for the monoclinic unit cell of CsCe2Cl7. ............................. 37 Figure 21. Comparison of measured temperature profile data with numerical model. ...................................... 40 Figure 22. Numerical model of transparent furnace with 10 mm thick nickel metal diaphram. ....................... 41 Figure 23. Numerical model of the transparent furnace with 3 mm thick Kaowool diaphragm. ....................... 42 Figure 24. Multipoint axial thermal profile. ............................................................................................................ 43 Figure 25. E-44 boule after growth .......................................................................................................................... 44 Figure 26. E-51 boule during and after growth ...................................................................................................... 45 Figure 27. The flattened and stable interface produced with the use of the diaphragm insert. ......................... 47 Figure 28. Inclusions bands in the boule prior to cooling and boule removed from ampoule. ........................... 47 Figure 29. Series of images from the timelapse of growth under minimal gradient. ........................................... 48 Figure 30. The E-68-2 boule at room temperature with and without annealing step prior to cooling. .............. 49 Figure 31. Boule E-68-3 at room temperature after a 48-hour high temperature annealing step. ..................... 50 Figure 32. The E-77 boule after growth. .................................................................................................................. 51 Figure 33. E-82 during growth. ................................................................................................................................ 52 Figure 34. Boule E-82 after growth. ......................................................................................................................... 53 Figure 35. The CsCe2Cl7 supercooling during the self-seeding process ................................................................ 54 Figure 36. E-91 during cooling. ................................................................................................................................ 55 Figure 37. E-91 boule after growth .......................................................................................................................... 57 Figure 38. Absolute light yield versus energy resolution of the 662 keV. ............................................................. 59 Figure 39. Gamma response spectra of CsCe2Cl7 under excitation by the 662 keV gamma .............................. 59 Figure 40. Non proportionality of response of CsCe2Cl7. ....................................................................................... 60 Figure 41. Transmittance measurement of CsCe2Cl7. ............................................................................................ 60 Figure 42. Radioluminescence spectra. .................................................................................................................... 61 Figure 43. Photoluminescence emission and excitation spectrum of CsCe2Cl7. ................................................... 62 Figure 44. ICP-MS compositional analysis of sample E-68-2 ................................................................................ 72

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List of Attachments Time lapse growth of E-68-1 with a large thermal gradient .................................... E-68-1_Large_Gradient.mov Time lapse growth of E-68-2 with a small thermal gradient .................................... E-68-2_Small_Gradient.mov Time lapse growth of E-82 with argon above melt ................................................ E-82_Argon_Above_Melt.mov Time lapse annealing and cooling of E-82 .......................................................E-82_Annealing_and_Cooling.mov Time lapse growth of E-91 with off stoichiometric composition.............................. E-91_Off_Stoichiometry.mov

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Objective Single crystalline inorganic scintillator materials are widely used as radiation detectors within industries spanning homeland security, natural resource exploration, and medical imaging. In recent years, several new promising scintillator compounds have been discovered. This work addresses a grand challenge in radiation detection for national security applications: the development of a gamma-ray sensitive detection material that can achieve ≤1% energy resolution at 662 keV at room temperature and can be produced in large volume at low to moderate cost. Large volume is considered to be 1 cubic inch but could in principle be grown in much larger volumes. As part of a broad project focused on identifying a small number of novel materials with potential for further development into growth processes approaching the goal of 1 cubic inch, experimental growth of single crystals from the melt in small volumes coupled with relevant material property research is a necessary requirement to assess issues affecting yield and development of basic protocols for practical growth at larger volumes. CsCe2Cl7, discovered recently as a promising intrinsic scintillator exhibits attractive performance with a fast decay time, good light yield, and emission wavelength suitable for efficient use with conventional photomultiplier tubes used in scintillator based detectors of X-ray and gamma-ray radiation. The hygroscopic nature of metal halides requires protection from ambient moisture to prevent degradation affecting scintillation performance and the very low hygroscopicity of CsCe2Cl7 in comparison with most other compositions in this class of materials is advantageous and provides additional motivation for investigation. Efforts to grow this material cite its tendency to crack upon cooling when using the Czochralski method which results in decreased crystalline quality thereby limiting the material’s scintillation performance and viability for future development. This thesis is comprised of a detailed comparison of crystal growth experiments aimed at improving the crystalline quality with respect to the growth procedures and an assessment of what improvements could be made through knowledge of the defect formation mechanisms governing polycrystallinity, inclusion formation, and cracking. Additionally, this study is intended to fill gaps in the scientific literature comprising the structural, thermodynamic, and thermophysical properties of CsCe2Cl7 relevant to single crystal synthesis and implementation into detector applications. Through the use of a transparent growth station commonly employed in research laboratories, direct and continuous observations of growth processes affecting crystal quality will be acquired to aid in the investigation. The thermal fields within growth stations will be measured and modeled to design suitable growth processes that maximize yield. In order to investigate the crystal structure and anisotropic thermal expansion behavior, single crystal Laue diffraction as well as high temperature x-ray powder diffraction analyses will be performed respectively. In addition to this, the thermodynamic behavior of melting and freezing will be investigated with differential scanning calorimetry. This method of investigation will provide a deeper understanding of material behavior pertaining to single crystal synthesis and can be readily applied to a wide array of compositions grown from the melt.

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Chapter 1: Introduction to Radiation Detection Electromagnetic radiation in the form of X-ray and gamma ray radiation lies outside the visible spectrum and is present in nature as naturally occurring cosmic radiation as well as the radioactive decay of naturally occurring elements found in the earth’s crust that are typically enriched for use as fuel in nuclear reactors. Other sources of potentially lethal radiation come from various man-made fission products produced in nuclear reactors and their daughter nuclides possibly used in nuclear weaponry. Regardless of the source, electromagnetic radiation with energies ranging from a few hundred of electron volts (eV) to several million electron volts (MeV) is highly penetrating and ionizing to the matter it interacts with, and because of this, there exists significant challenges to the safe containment and shielding of radioactive material with regards to the hazards it presents to biological organisms. As a consequence of the highly penetrating nature of electromagnetic radiation, the accurate detection and identification of sources is also made challenging. However, it is also this quality that presents many important uses to society and various means of utilizing electromagnetic radiation have been developed over the past century.

1.1 Application The subject of this study pertains to research and development of detection materials for use in homeland security applications where efficient detection and identification of nuclear radiation is the key function. Detectors of this type are implemented at thousands of locations ranging from security checkpoints at airport or ground transport terminals, border crossings, population centers, as well as weigh stations in order to identify radioactive contraband being transported or shipped by individuals aimed at using them for illicit purposes such as in nuclear weaponry for state warfare or acts of terrorism. Portable configurations of detectors ranging from ground or air vehicle mounted systems to backpack and handheld units capable of identifying radioisotopes in various conditions are required for inspections of areas suspected of processing or harboring illegal nuclear radioactive contraband as a method to enforce nuclear non-proliferation agreements around the globe. As a result, detectors used in this application must be supplied in large volumes possessing a marriage of low cost of production with detector performance. Presently, the best technology currently available for radionuclide identification is based upon the use of high purity germanium (HPGe) semiconductor based detectors. However, the ultra-high purity germanium required for production and cryogenic operating temperatures make this technology prohibitively expensive for practical deployment for use in national security applications. As a result, cheaper alternatives are needed which still provide functional radionuclide identification and scintillator based detectors provide a significantly lower cost of production and operation (1).

1.2 Scintillator Based Detectors

Inorganic single crystal scintillators have been in use for decades to detect X-ray and gamma-ray radiation as they provide an efficient means of converting the high energy electromagnetic radiation into visible photons of light. Scintillators absorb individual quanta of high energy X-rays or gamma rays and respond through emission of a pulse of many quanta of lower energy electromagnetic radiation in form of visible photons. The visible photons must be converted into electrons in order to be quickly measured and recorded by observation devices and thus belong to the indirect detector category. This is typically accomplished through coupling the scintillator to a photo-multiplier-tube (PMT) whereby scintillation photons which strike a photocathode cause an electron to be emitted through the photoelectric effect. The 2

photoelectron is then accelerated by electrostatic potential toward several dynodes in the PMT which multiply it through sequential stages into millions of electrons that ultimately flow to the anode producing a current pulse signal for further processing by amplifiers, timing circuitry, and a multichannel analyzer (2). Through this process, the number of emitted scintillation photons as a response to ionizing radiation is proportional to the current signal registered in the aforementioned electronics. For spectroscopy applications requiring fast and efficient radionuclide identification, the scintillator material must possess in general: • • • •

A high density and effective Z number to provide good stopping power and probability of interaction with high energy X-ray and gamma-ray radiation. A high light yield to produce enough photoelectrons for subsequent processing by appropriate electronic circuitry. A high energy resolution or ability to resolve different energies of ionizing radiation which provide the signature of the radionuclide source(s). Cost effective means of production of single crystals to provide large volumes of material for various detector configurations.

Many inorganic crystalline scintillators have been discovered yet no single one offers the best performance in all categories for security applications. Consequently, amongst all crystalline scintillator materials, the metal halides possess a marriage of high light yield and best achievable energy resolution to date (3) and thus are well suited for spectroscopy despite lower densities typically under 5.5 g/cm3. In comparison with oxide based scintillators that possess higher densities, only a few have demonstrated comparable energy resolution and light yield to metal halides such as cerium activated yttrium aluminum perovskite (YAlO3:Ce or YAP) yet their higher melting point (~1900°C) introduces considerable cost to production of large volumes of single crystals and thus cannot compete with the cost of production of metal halides possessing a lower melting point (typically 1 cubic inch) single crystals. Cerium activated lanthanum bromide (LaBr3:Ce) as well as the intrinsic scintillator cerium bromide (CeBr3) both offer very good energy resolution of approximately 3% and 4% at 662 keV respectively yet because of their similar hexagonal structure, each have a weak cleavage plane that makes growth of uncracked single crystals difficult and costly. Over many years of research, the growth processes developed by company SaintGobain has yielded 3.5 inch diameter boules (7, 8). Because of the fragility and tendency to crack during growth, research into strengthening the material through aliovalent doping in parts per million (ppm) levels has achieved some improvement (9) yet costs of production remain prohibitively high for use in large detectors for security applications. 3

One of the best performing scintillators in terms of energy resolution is europium doped strontium iodide (SrI2:Eu) which can achieve